Boiling water nuclear reactors are known. In such reactors, fuel bundles contain the fuel which undergoes the nuclear reaction. Typically, such fuel bundles include a supporting lower tie plate for holding an array of vertically upstanding fuel rods and permitting the entry of water moderator. A channel extends from the bottom tie plate around the fuel rods to an upper tie plate at the top of the fuel bundle. This upper tie plate permits the exit of both water and generated steam.
Interior of the fuel bundle channel and around each of the fuel rods at selected elevations, so-called fuel rod spacers are utilized. It is the function of these fuel rod spacers to prevent the fuel rods from coming into abrading contact one with another under the fluid flow dynamic forces of steam generation as well as to maintain the designed nuclear spacing of the fuel rods for maximum nuclear efficiency. Accordingly, it is standard practice to include between 5 to 9 fuel rod spacers--and usually 7 fuel rod spacers at usually equal vertical intervals within each fuel bundle.
Operation of the fuel bundles can be simply described. Water moderator enters the fuel bundle at the lower tie plate where the water assists the nuclear reaction in two ways. First, the reaction produces fast neutrons which must be moderated or slowed down to their thermal state where their velocity corresponds to their thermal energy due to the moderator thermal energy for continuing the reaction. Secondly, the water coolant permits the fuel bundles to generate steam from which the power generated by the reactor can be extracted. Initially, and in the lower portion of the fuel bundle, water enters--with no steam being present. As the moderator rises up through the fuel bundle, increasing amounts of steam are generated. These increasing amounts of steam cause the upper portion of the fuel bundle to be known as the upper two phase region--these two phases being water and steam. Finally, the water and generated steam exit the upper tie plate of the fuel bundle.
When boiling water reactors having fuel bundles were originally introduced, the fuel channel size was fixed with the design of the original reactor vessel. Hence, most fuel bundles for most boiling water reactors have a length dimension in the order of 160 inches and a square cross section which is approximately 51/4.times.51/4 fitting snugly in a confining channel.
Within the constraining dimensions of the fuel bundles, there has been a tendency to change fuel bundle design. Such change has included the density of the fuel rods within the bundle array utilized within the fuel bundle as well as the types of fuel rod spacers utilized. Some explanation of the reason behind the design changes is instructive.
Originally, fuel rods--long sealed tubes of Zircaloy cladding surrounding fuel pellets within an inert compressed gas bath--were in arrays of 7 by 7 within the fixed 51/4 inch by 51/4 inch fuel channel cross section. Among other factors, such fuel bundles were constrained by their maximum linear heat generation rate. Maximum linear heat generation rate is the maximum amount of power per unit length that any fuel rod can safely output anywhere within the fuel bundle. Where this maximum linear heat generation rate is exceeded, the sealed integrity of the fuel bundle can be threatened at least along the length of the fuel rod where the maximum linear heat generation rate has been exceeded. This threat is due to excessive strain which can lead to degradation of the metallurgical qualities of the containing and sealed fuel rod cladding.
Maximum linear heat generation rate limitations caused fuel designs to utilize relatively denser arrays of fuel rods which have greater total linear feet of rod length in a bundle. These dense arrays have included 8 by 8 arrays, 9 by 9 arrays and even higher density arrays such as 10 by 10. These higher density arrays accommodate fuel rods of smaller diameter which more rapidly transfer their heat to the surrounding coolant. Such smaller diameter less massive fuel rods have the fortunate effect having a maximum linear heat generation rate which can be tolerated within the fuel rods. Thus the modern tendency is to include in fuel bundle designs, fuel rod arrays having higher densities.
At the same time, so-called critical power is also a limitation on fuel bundle design. This limitation is related to the phenomena known as "boiling transition." Simply summarized, in the upper two phase region of a fuel bundle, water typically forms a coating over the surface of the fuel rods. Steam is generated from the surface of this film of liquid. Through a complex process--which process is not entirely understood--this film of water maintains itself covering the exterior cladding of the fuel rods--provided that the so-called critical power limitation of the fuel rods is not exceeded.
When the critical power limitation of the fuel rod is exceeded, the fuel rod no longer has a continuous coating of liquid over its surface. Instead--and through the process known as "transition boiling"--local interruptions of the water coating occur. As a consequence, the local temperature of the fuel rod cladding rises at the point of transition boiling. The integrity of the fuel rod is locally threatened due to possible metallurgical break down of the cladding wall at the point of transition boiling.
It has been found that so-called ferrule spacers can have a beneficial effect on critical power. To understand this beneficial effect--and its limitations on high density arrays of fuel rods, both the construction and the effect of such ferrule spacers should be summarized.
Regarding the construction of ferrule spacers, each fuel rod at the elevation of the spacer is surrounded by a cylinder of metal--usually an alloy known as Zircaloy having a relatively small cross section for the absorption of neutrons. Each ferrule defines a plurality of stops against which the fuel contained fuel rod can be spring biased to maintain the designed spacing of the fuel rods. Springs to effect the spring bias of the fuel rods are usually mounted within the ferrules. Thus the spacers not only prevent the side-to-side abrading contact of the fuel rods but serve to maintain the fuel rods in their designed spaced apart relation for maximum nuclear and thermal efficiency.
The effect of such ferrule spacers on the critical power of such fuel rods can be set forth. Specifically, and after the steam/water mixture in the upper two phase region of the fuel bundle has passed through a ferrule of a ferrule spacer, the water coating on the outside of the fuel rod tends to be restored. By controlling the vertical spacing interval either between the first and second spacers from the top of the fuel bundle--or the second and third spacers from the top of the fuel bundle, ferrule spacers can improve the critical power limitation in nuclear fuel bundles.
Unfortunately, utilizing ferrule spacers in high density fuel rod arrays is not without problems. Specifically, and where for example a 9 by 9 array of fuel rods is utilized, each of the fuel rods must be surrounded by its own discrete ferrule. Thus, across the relatively narrow 51/4 inch by 51/4 inch section of a fuel bundle containing an array of 9 by 9 fuel rods, no less than 18 ferrule walls must be disposed along with each of the 9 fuel rods and sufficient clearance for the passage of water being generated into steam between the fuel rods and the ferrules. In other words, where the number of fuel rods extending across a fuel bundle goes up, so does the number of ferrule walls required for maintaining a ferrule rod spacer.
As the number of required ferrule walls increases together with the number of fuel rods to be surrounded, and the available cross sectional dimension (about 51/4 inches by about 51/4 inches) remains unchanged, the clearance between the fuel rods and ferrules shrinks. Gradually, this clearance approaches that point where the gap between the ferrules and the fuel rods become so narrow that this gap becomes a likely candidate for the buildup of resident debris within the reactor as well as a likely location for corrosion.
An additional problem arises because of the close packing of fuel rods and ferrules in high density arrays; the ferrules occupy a greater fraction of the fuel bundle cross section, and cause a greater pressure drop as the coolant passes through the spacer.
Therefore, the use of ferrule spacers in high density arrays has been limited.
The invention disclosed herein addresses the two problems described above; namely the small gap between the fuel rod and ferrule, and the increased pressure drop which occurs in high density fuel rod arrays.